A boy celebrating his birthday and his young sister lost both their parents when a customer annoyed by a waiter fired several shots inside the busy restaurant, in Songkhla on Sunday night.

The shooting occurred at Khrua Paowana restaurant on Nuan Kaew Uthit Road about 10pm on Sunday.

According to eyewitnesses, the gunman was Akarapon Pamchamnong, a 50-year-old defence volunteer of Hat Yai district, police said.

He came to the restaurant alone to drink. After a while, he stepped outside to smoke on the roadside. A waiter who thought that he had finished drinking cleared the table.

When Mr Akarapon saw that, he became annoyed and argued with the waiter and the restaurant owner and then drove away. 

He later returned, resumed the argument, drew out a gun and fired it repeatedly.

Two customers were killed - Sitthipong Siriphan, 48, and his wife Orachon Ieankham, 45, both shot in the head.

The couple had brought their young children to the restaurant for a birthday party for their 12-year-old son. The boy was unhurt but his younger sister suffered a wound to her head.

Two other people were wounded. 

The gunman surrendered to police.